1. Field of the Invention
The present invention relates to optical projection systems and, in particular, to a system and method for automatically controlling image registration, brightness and gamma correction in a full-color light valve projection system.
2. Discussion of the Prior Art
In a typical full-color projection system, illumination light from a xenon arc lamp is directed toward a liquid crystal light valve system through a polarizer/reflector optical system. The illumination light first passes through a first beam-splitting polarizing prism that pre-polarizes the light. S-polarized light is rejected from the system at this point and p-polarized light is transmitted. A second identical beam-splitting polarizing prism, oriented at 90.degree. with respect to the first prism, "sees" the light incident from the first prism as s-polarized and, therefore, reflects it toward the liquid crystal light valves. The s-polarized light from the second prism is first directed through a blue reflecting dichroic filter. Green and red wavelengths are transmitted by this filter and next encounter a red reflecting dichroic filter. This second filter reflects the red light and transmits the green. The green light is then transmitted directly to a "green" liquid crystal light valve. The incoming red and blue light beams are each reflected to their respective liquid crystal light valves. Thus, the s-polarized illumination light leaving the second prism is split into three individual primary color illumination beams, each of which enters a separate reflective liquid crystal light valve. The light from an image on an individual CRT associated with each color channel is directed to the photoconductive region of its associated light valve. Thus, on reflection by a liquid crystal light valve, the outgoing light contains a polarized, intensity-modulated, single-color, optical image that is a replica of the image on the CRTs. The outgoing light from each liquid crystal light valve then retraces its way back through the color filter assembly. The red and green modulated beams are recombined at the red reflecting filter and these are then recombined with the blue modulated beam at the blue reflecting filter. The recombined light from the three color channels now shares the same common optical axis as it did before intensity modulation. This recombined light passes back through the second polarizing prism which transmits p-polarized light and reflects s-polarized light back toward the arc lamp illumination light source. The light transmitted by the second prism now consists of intensity modulated light containing the full-color light image from the CRTs. This primary projection image is passed through a wide angle projection lens which directs it toward a screen for display.
A color projection display system of the type just described is disclosed by Ledebuhr "Full-Color Single-Projection-Lens Liquid-Crystal Light-Valve Projector", SID 86 Digest, p. 381.
For a system such as that described by Ledebuhr to provide a high quality, high resolution display image, it is necessary to monitor the displayed image, detect characteristics of that image and adjust the projection system accordingly to obtain the desired quality display.
For example, to maintain accurate registration of the three primary color projected images over the entire display screen area, the CRTs of system such as that described by Ledebuhr contain focus and astigmatism coils for spot shape programming, as well as additional deflection coils which are used to dynamically shape the three images. While this produces an initial registered image on the display area, thermal and electrical drifts can cause image misregistration.
In order to reduce the drift to zero, the system described by Ledebuhr includes a feedback loop which is used to maintain the registration of the primary color images. Three position-sensitive photodetectors are mounted at the display screen, outside the display area, to monitor the position of three projected target patterns that are generated during the vertical blanking interval of the display. Position signals from these three sensors are used to provide correction signals to the CRT deflection coils, which independently position the location of each color image.
It is also necessary to correct for gamma distortions within the projection system. Gamma is the characteristic that mathematically describes the relationship between the cathode current density and the amount of light produced by a given CRT. A linear intensity increment from the CRT signal generator will produce a non-linear change in visible brightness in the projected display image. In the past, non-linear analog amplifiers have been provided in the CRT video circuitry to correct for gamma distortions. According to another approach, the signal voltage for each color component for a given pixel value is set via digital values stored in a color look-up table. That signal is then converted to a cathode drive current for the CRT.
It is also necessary to correct the projected display image for brightness non-uniformities in the display output. In the past, this has been accomplished by summing the horizontal and vertical rate waveforms into a video signal to correct for brightness variations.